Driver for switching element and control system for rotary machine using the same

ABSTRACT

In a driver, a changing module changes a rate of discharging the control terminal of a switch at least between a first value and a second value lower than the first value. A measuring module measures a value of a parameter as a function of a current flowing through the conductive path of the switch during a drive signal being in an on state. A control module controls the changing module, as a function of the value of the parameter, to select the first value or the second value as the rate of discharging the control terminal of the switch upon the drive signal directing a change from the on state of the switch to an off state thereof. The control module discharges the control terminal of the switch using the selected value as the rate of discharging the control terminal of the switch.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and claims the benefit of priority fromJapanese Patent Application 2012-003741 filed on Jan. 12, 2012, thedisclosure of which is incorporated in its entirety herein by reference.

TECHNICAL FIELD

The present disclosure relates to drivers for selectively storing anddissipating an electrical charge on and from the control terminals ofvoltage-controlled switching elements to selectively turn on and off thevoltage-controlled switching elements. The present disclosure alsorelates to control systems for rotary machines using the drivers.

BACKGROUND

An example of active gate control for changing the rate of dischargingthe gate of an insulated-gate transistor, such as an IGBT(Insulated-Gate Bipolar Transistor), within a period from the start ofdissipating charge stored in the gate to completion of the dissipationis known in, for example, Japanese Patent Publication No. 3373704,referred to as a first patent document. The known active gate controlrequires a differentiating circuit for outputting a signal indicative ofa derivative value of a collector current flowing through aninsulated-gate transistor. The known active gate control changes thedischarge path connected to the gate of the insulated-gate transistorfrom a low-resistance discharge path to a high-resistance discharge pathbased on a result of comparison between the level of an output signalfrom the differentiating circuit and that of a reference signal.Specifically, the known active gate control is designed to increase theresistance of the discharge path connected to the gate of theinsulated-gate transistor if the rate of reduction of the collectorcurrent is equal to or higher than a predetermined value. The knownactive gate control reduces an increase of a surge produced based on therate of reduction of the collector current while reducing switchingloss.

Another example of the active gate control is known in, for example,Japanese Patent Publication No. 3339311, referred to a second patentdocument. The active gate control known in the second patent documentprovides a driver for a voltage-controlled switching element, such as anIGBT; the driver includes two discharge paths connected to thevoltage-controlled switching element and having different resistancevalues. The active gate control known in the second patent documentachieves the same effect as that known in the first patent document.

SUMMARY

Even if a surge is produced within a period during which aninsulated-gate transistor is shifted from an on state to an off state,there may be a certain margin of voltage between an actually appliedvoltage across both ends, i.e. the collector and emitter, of aconductive path of the insulated-gate transistor and an acceptable upperlimit therefor. In this case, if the timing to change the rate ofdischarging the gate of the insulated-gate transistor is uniformlydetermined based on the output signal from the differentiating circuitdisclosed in the first patent document, it may be difficult to increasethe switching speed of the insulated-gate transistor although there isroom for increase in the switching speed using the margin. This mayresult in a decrease of the effect of reducing switching loss.

In view of the circumstances set forth above, one aspect of the presentdisclosure seeks to provide drivers for selectively storing anddissipating an electrical charge on and from the control terminals ofvoltage-controlled switching elements, which are designed to solve theproblem set forth above.

Specifically, an alternative aspect of the present disclosure aims toprovide such drivers, which are capable of sufficiently maintaining theeffect of decreasing switching loss using active gate control. In otherwords, the alternative aspect of the present disclosure aims to providesuch drivers, which are capable of preventing a decrease in the effectof reducing switching loss.

According to a first exemplary aspect of the present disclosure, thereis provided a driver for selectively performing a charging task and adischarging task for an on-off control terminal of a voltage-controlledswitch having a conductive path, according to a drive signal directingselectively an on state and an off state of the voltage-controlledswitch, thus setting the voltage-controlled switch to one of the onstate and the off state. The driver includes a discharging-rate changingmodule adapted to change a rate of discharging the on-off controlterminal of the voltage-controlled switch at least between a first valueand a second value lower than the first value. The driver includes ameasuring module configured to measure a value of a parameter as afunction of a current flowing through the conductive path of thevoltage-controlled switch during the drive signal being in the on state.The driver includes a discharging control module configured to:

control the discharging-rate changing module, as a function of the valueof the parameter, to select one of the first value and the second valueas the rate of discharging the on-off control terminal of thevoltage-controlled switch upon the drive signal directing a change fromthe on state of the voltage-controlled switch to the off state thereof;and

discharge the on-off control terminal of the voltage-controlled switchusing the selected one of the first value and the second value as therate of discharging the on-off control terminal of thevoltage-controlled switch.

A surge produced within the period during which the voltage-controlledswitch is changed from the on state to the off state has acharacteristic that, the higher the current flowing through theconductive path of the voltage-controlled switch is during the on stateof the voltage-controlled switch, the higher the level of the surge is.This is because, the higher the current, referred to as a main current,flowing through the conductive path is when the voltage-controlledswitching element is in the on state, the higher the rate of reductionof the main current is. Focusing on this characteristic, if the maincurrent flowing through the voltage-controlled switching element is alower value when the voltage-controlled switching element is in the onstate, there is a certain margin of voltage between an actually appliedvoltage across the conductive path of the voltage-controlled switchingelement and an acceptable upper limit therefor within the period duringwhich the voltage-controlled switch is changed from the on state to theoff state.

In view of this point, the driver according to the first exemplaryaspect of the present disclosure is configured to control thedischarging-rate changing module, as a function of the value of theparameter correlating with the main current, to select one of the firstvalue and the second value as the rate of discharging the on-off controlterminal of the voltage-controlled switch upon the drive signaldirecting a change from the on state to the off state. That is, thedriver according to the first exemplary aspect of the present disclosureadjusts the rate of discharging the on-off control terminal of thevoltage-controlled switch according to the value of the parametercorrelating with the main current so as to, for example, increase theswitching speed of the voltage-controlled switch, making it possible tomaintain, at a high level, the effect of decreasing switching lossthereof.

Note that discharging the gate of the voltage-controlled switchingelement means, for example, dissipating positive change from the gate ofthe voltage-controlled switching element, and storing negative charge onthe gate of the voltage-controlled switching element.

According to a second exemplary aspect of the present disclosure, thereis provided a control system for controlling a rotary machine. Thecontrol system includes a converter equipped with at least one pair offirst voltage-controlled switching elements connected in series, each ofthe first voltage-controlled switching elements having a conductive pathand an on-off control terminal. The converter includes an inverterequipped with at least one pair of second voltage-controlled switchingelements connected in series, each of the second voltage-controlledswitching elements having a conductive path and an on-off controlterminal. The converter includes a driver for selectively performing acharging task and a discharging task for the on-off control terminal ofeach of the voltage-controlled switching elements, according to a drivesignal directing selectively an on state and an off state of thevoltage-controlled switch to thereby boost a DC voltage inputted to theconverter, and invert the boosted DC voltage into an AC voltage to besupplied to the rotary machine. The driver for each of the first andsecond voltage-controlled switching elements includes a discharging-ratechanging module adapted to change a rate of discharging the on-offcontrol terminal of a corresponding one of the first and secondvoltage-controlled switching elements at least between a first value anda second value lower than the first value. The driver for each of thefirst and second voltage-controlled switching elements includes ameasuring module configured to measure a value of a parameter as afunction of a current flowing through the conductive path of acorresponding one of the first and second voltage-controlled switchingelements during the drive signal being in the on state. The driverincludes a discharging control module configured to:

control the discharging-rate changing module, as a function of the valueof the parameter, to select one of the first value and the second valueas the rate of discharging the on-off control terminal of acorresponding one of the first and second voltage-controlled switchingelements upon the drive signal directing a change from the on state of acorresponding one of the first and second voltage-controlled switchingelements to the off state thereof; and

discharge the on-off control terminal of a corresponding one of thefirst and second voltage-controlled switching elements using theselected one of the first value and the second value as the rate ofdischarging the on-off control terminal thereof.

The converter and the inverter of the control system according to thesecond exemplary aspect of the present disclosure include a driveraccording to the first exemplary aspect of the present disclosure foreach of the first and second voltage-controlled switching elements. Forthis reason, the control system achieves the same technical effect asthe driver according to the first exemplary aspect of the presentdisclosure does.

The above and/or other features, and/or advantages of various aspects ofthe present disclosure will be further appreciated in view of thefollowing description in conjunction with the accompanying drawings.Various aspects of the present disclosure can include and/or excludedifferent features, and/or advantages where applicable. In addition,various aspects of the present disclosure can combine one or morefeature of other embodiments where applicable. The descriptions offeatures, and/or advantages of particular embodiments should not beconstrued as limiting other embodiments or the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects of the present disclosure will become apparent from thefollowing description of embodiments with reference to the accompanyingdrawings in which:

FIG. 1 is a view schematically illustrating an overall configuration ofa control system for a motor-generator according to a first embodimentof the present disclosure;

FIG. 2 is a circuit diagram schematically illustrating each drive unitof the control system illustrated in FIG. 1;

FIG. 3 is a graph schematically illustrating measurement results of howa gate voltage, a collector-emitter voltage, a sense voltage, and acollector current vary after the start of discharging the gate of aswitching element due to the turnoff of the switching element;

FIG. 4 is a graph schematically illustrating measurement results of howthe gate voltage, the collector-emitter voltage, the sense voltage, andthe collector current vary after the start of charging the gate of aswitching element due to the turn-on of the switching element;

FIG. 5 is a flowchart schematically illustrating an example of acharging and discharging routine carried out by each drive unit of thecontrol system illustrated in FIG. 1;

FIG. 6 is a timing chart schematically illustrating an example ofspecific operations of variables correlated with the charging anddischarging routine using active gate control;

FIG. 7 is a timing chart schematically illustrating an example ofspecific operations of the variables correlated with the charging anddischarging routine while the active gate control is not used;

FIG. 8 are first and second graphs, the first graph schematicallyillustrating a first measured correlation between collector current andsurge voltage for each switching element according to the firstembodiment and a second measured correlation therebetween according to aknown example, the second graph schematically illustrating a thirdmeasured correlation between collector current and switching loss foreach switching element according to the first embodiment and a fourthmeasured correlation therebetween according to the known example;

FIG. 9A is a timing chart schematically illustrating an example ofspecific operations of variables correlated with a charging anddischarging routine using active gate control according to a secondembodiment of the present disclosure;

FIG. 9B is a flowchart schematically illustrating an example of a partof the charging and discharging routine according to the secondembodiment;

FIG. 10A is a timing chart schematically illustrating an example ofspecific operations of variables correlated with a charging anddischarging routine using active gate control according to a thirdembodiment of the present disclosure;

FIG. 10B is a flowchart schematically illustrating an example of a partof the charging and discharging routine according to the thirdembodiment;

FIG. 11A is a timing chart schematically illustrating an example ofspecific operations of variables correlated with a charging anddischarging routine using active gate control according to a fourthembodiment of the present disclosure;

FIG. 11B is a flowchart schematically illustrating an example of a partof the charging and discharging routine according to the fourthembodiment;

FIG. 12 is a timing chart schematically illustrating an example ofspecific operations of variables correlated with a charging anddischarging routine using active gate control according to a fifthembodiment of the present disclosure;

FIG. 13 is a timing chart schematically illustrating an example ofspecific operations of the variables correlated with the charging anddischarging routine while the active gate control is not used accordingto the fifth embodiment; and

FIG. 14 is a flowchart schematically illustrating an example of thecharging and discharging routine according to the fifth embodiment.

DETAILED DESCRIPTION OF EMBODIMENT

Embodiments of the present disclosure will be described hereinafter withreference to the accompanying drawings. In the embodiments, like partsbetween the embodiments, to which like reference characters areassigned, are omitted or simplified to avoid redundant description.

First Embodiment

Referring to FIG. 1, there is illustrated a three-phase motor-generatoras an example of rotating machines, referred to simply as a“motor-generator” 10, installed in, for example, a motor vehicle as amain engine according to the first embodiment. The motor-generator 10 ismechanically coupled to driving wheels (not shown) of the motor vehicle.

For example, as the motor-generator 10, a brush-less DC motor, i.e. athree-phase SM (Synchronous Motor), is used.

The motor-generator 10 is made up of, for example, a rotor including amagnetic field and a stator including three-phase windings, i.e. U-, V-,and W-phase windings. The rotor of the motor-generator 10 is rotatedbased on magnetic interaction between the magnetic field of the rotorand a rotating field generated by the three-phase windings when thethree-phase windings are energized. For example, the three-phasewindings (U-, V-, and W-phase windings) each have one end connected to acommon junction (neutral point) and the other end to a separate terminalin, for example, a star-configuration.

In FIG. 1, there is also illustrated a control system 100 forcontrolling the motor-generator 10. The control system 100 is equippedwith an inverter INV, a converter CNV, a high-voltage battery 12 as anexample of DC power sources, drive units, i.e. drivers, DU, a controlunit 14, a low-voltage battery 16, and an interface 18.

To the motor-generator 10, the high-voltage battery 12 is electricallyconnected via the inverter INV and the converter CNV. The high-voltagebattery 12 has a terminal voltage of, for example, 288 V thereacross.

The converter CNV includes a capacitor C, a pair of series-connectedswitches, in other words, switching elements Scp and Scn, a pair offlywheel diodes Dcp and Dcn, and a reactor L. The capacitor C isconnected in parallel to the inverter INV, and the series-connectedswitching elements Scp and Scn are connected in parallel to thecapacitor C. The flywheel diodes Dcp and Dcn are connected inantiparallel to the corresponding switching elements Scp and Scn,respectively. One end of the reactor L is connected to both the positiveterminal of the high-voltage battery 12 and the connection point betweenthe switching elements Scp and Scn. One end of the series-connectedswitching elements Scp and Scn of the converter CNV is connected to thepositive DC input line of the inverter INV, and the other end thereof isconnected to the negative DC input line of the inverter INV. Thenegative DC input line of the inverter INV is connected to the negativeterminal of the battery 12.

The converter CNV is operative to convert the terminal voltage of thehigh-voltage battery 12 into a voltage higher than the terminal voltageof the high-voltage battery 12, and output the boosted voltage as anoutput DC voltage thereof across the capacitor C. The predeterminedupper limit of the step-up of the terminal voltage by the converter CNVis set to a predetermined high voltage, such as 666 V.

The inverter INV is designed as a three-phase inverter. The inverter INVis provided with three pairs of series-connected high- and low-side(upper- and lower-arm) switches, in other words, switching elements Supand Sun, Svp and Svn, and Swp and Swn. The inverter INV is also providedwith flywheel diodes D*# (*=u, v, w, #=p, n) electrically connected inantiparallel to the corresponding switching elements S*# (*=u, v, w,#=p, n), respectively.

In the first embodiment, as the switching elements S*# (*=u, v, w, #=p,n), IGBTs are respectively used.

When power MOSFETs are used as the switching elements S*# (*=u, v, w,#=p, n), intrinsic diodes of the power MOSFETs can be used as theflywheel diodes, thus eliminating the flywheel diodes.

The three pairs of switching elements are parallelly connected to eachother in bridge configuration. A connecting point through which each ofthe switching elements S*p (*=u, v, w) is connected to a correspondingone of the S*n (*=u, v, w) in series is connected to an output leadextending from the separate terminal of a corresponding one of theU-phase winding, V-phase winding, and W-phase winding. One end of theseries-connected switching elements of each of the three pairs, such asthe collector of the corresponding high-side switching element, isconnected to the positive terminal of the high-voltage battery 12 viathe positive DC input line. The other end of the series-connectedswitching elements of each of the three pairs, such as the emitter ofthe corresponding low-side switching element, is connected to thenegative terminal of the high-voltage battery 12 via the negative DCinput line.

For example, the control unit 14 operates on a power-supply voltage,lower than the terminal voltage across the high-voltage battery 12,supplied from the low-voltage battery 16. Thus, the control unit 14 andthe low-voltage battery 16 constitute a low voltage system. In contrast,the motor-generator 10, the converter CNV, the inverter INV, and thehigh-voltage battery 12 constitute a high voltage system.

The interface 18 is provided with insulation members, such asphotocouplers 18 a provided for the respective switching elements S*# ofthe inverter INV and converter CNV. Each of the photocouplers 18 a iscomprised of a photodiode and a phototransistor. The photocouplers 18 aare configured to enable communications between the high and low voltagesystems while establishing electrical insulation therebetween.Specifically, each of the photocouplers 18 a is configured to enable thecontrol unit 14 to control a corresponding one of the switching elementsS*# while establishing electrical insulation between the control unit 14and a corresponding one of the switching elements S*#.

The control unit 14 is designed to individually drive the inverter INVand the converter CNV to thereby control a controlled variable of themotor-generator 10, such as an output torque of the motor-generator 10.

Specifically, the control unit 14 is designed to individually send drivesignals gcp and gcn to the drive units DU provided for the respectiveswitching elements Scp and Scn, thus individually turning on or off therespective switching elements Scp and Scn. The control unit 14 is alsodesigned to individually send drive signals gup, gun, gyp, gvn, gwp, andgwn to the drive units DU provided for the respective switching elementsSup, Sun, Svp, Svn, Swp, and Swn, thus individually turning on or offthe respective switching elements Sup, Sun, Svp, Svn, Swp, and Swn. Theindividual turn-on or off of the respective switching elements Sup, Sun,Svp, Svn, Swp, and Swn convert the output DC voltage across thecapacitor C into an AC voltage, and supply the AC voltage to themotor-generator 10.

Each of the drive signals g*# has a predetermined duty cycle, i.e. apredetermined ratio of on duration to the total duration of eachswitching cycle for a corresponding one of the switching elements S*#(see FIG. 1).

Specifically, the control unit 14 is designed to complementarily turn onthe high- and low-side switching elements S*# for each leg (phase) viathe corresponding drive units DU according to the corresponding drivesignals g*#. In other words, the control unit 14 is designed toalternately turn on the high-side switching element S*p of one leg(phase) and the low-side switching element S*n of the same leg (phase).This drive alternately closes the conductive path between the collectorand emitter of the high-side switching element S*p of one leg and theconductive path between the collector and emitter of the high-sideswitching element S*n of the same leg.

Next, an example of the circuit structure of each drive unit DU providedfor a corresponding one switching element S*# will be described withreference to FIG. 2.

Referring to FIG. 2, the drive unit DU is comprised of a drive IC 20 ona chip, resistors 24, 34 a, 34 b, 38, and 42, and an off-state holdingswitching element 40.

The drive IC 20 has terminals T1 to T13, a drive controller 22, a powersource 26, a series regulator 28, a resistor 30, a charge switchingelement 32, a first discharge switching element 36 a, a second dischargeswitching element 36 b, and a soft-turnoff switching element 44. Thedrive IC 20 also has a comparator 46, a reference power source 48, and adelay unit 50. As the charging switching element 32, a P-channel MOSFETis used, and as each of the switching elements 36 a, 36 b, 40, and 44,an N-channel MOSFET is used.

An output terminal of the power source 26 is connected to the drivecontroller 22 via the terminal T2, the resistor 24, i.e. the pull-upresistor 24, and the terminal T1. The power source 26 has, for example,a terminal voltage of 5 V. To the terminal T1, the secondary side, i.e.the collector of the phototransistor, of the photocoupler 18 a isconnected. The emitter of the phototransistor of the photocoupler 18 ais grounded. The primary side of the photocoupler 18 a, i.e. thephotodiode, is connected to the control unit 14.

That is, the phototransistor of the photocoupler 18 a of each switchingelement S*# is OFF if a drive signal g*# with a low level outputted fromthe control unit 14 is inputted to the photocoupler 18 a. This causesthe resistor 24 to pull up the potential of the terminal T1 of the driveIC 20 to the terminal voltage of the power source 26. Otherwise, if adrive signal g*# with a high level outputted from the control unit 14 isinputted to the photocoupler 18 a, the phototransistor of thephotocoupler 18 a of each switching element S*# is ON. This causes theterminal T1 of the drive IC 20 to be grounded via the phototransistor ofthe photocoupler 18 a.

That is, if the photocoupler 18 a is OFF, the drive signal Vi*# with ahigh voltage level is inputted to the drive controller 22 via theterminal T1, and otherwise if the photocoupler 18 a is ON, the drivesignal Vi*# with a low voltage level is inputted to the drive controller22 via the terminal T1. If the drive signal Vi*# directs the high level,i.e. ON, the drive controller 22 is configured to carry out adischarging task for the on-off control terminal, i.e. gate, of acorresponding switching element S*# described later. Otherwise, if thedrive signal Vi*# directs the low level, i.e. OFF, the drive controller22 is configured to carry out a charging task for the gate of acorresponding switching element S*# described later. Note that the drivesignal Vi*# with the high level will be referred to as an off command,and the drive signal Vi*# with the low level will be referred to as anon command.

In addition, note that the drive signal g*p and the drive signal Vi*pfor the high-side switching element S*p of each leg (phase) and thedrive signal g*n and the drive signal Vi*n for the low-side switchingelement S*n of a corresponding one leg (phase) are complementary signalstherebetween. In other words, the high-side switching element S*p of oneleg and the low-side switching element S*n of the same leg arecomplementarily turned on.

To the terminal T3 of the drive IC 20, a voltage Vfb of a power sourcefor the drive unit DU is applied. For example, as the power source, aflyback converter 46 installed in, for example, the interface 14 can beused. The flyback converter 46 is comprised of a transformer 46 a and adiode 46 b; the transformer 46 a includes a primary winding and asecondary winding. Magnetic energy charged in the primary winding basedon the voltage across the low-voltage battery 16 induces a voltageacross the secondary winding, and power based on the voltage inducedacross the secondary winding is supplied via the diode 46 b and theterminal T3 of the drive IC 20 to the series regulator 28.

The series regulator 28 is operative to step down the voltage Vfbsupplied from the flyback converter 46 to a voltage Vom to be applied tothe gate of the switching element S*#.

An output terminal of the series regulator 28 is connected to one end ofthe resistor 30 via the terminals T4 and T5 of the drive IC 20. Theother end of the resistor 30 is connected to the source of the chargeswitching element 32 in series, and the drain of the charge switchingelement 32 is connected to the gate of the switching element S*# via theterminal T6 of the drive IC 20. The gate of the charge switching element32 is connected to the drive controller 22.

The gate of the switching element S*# is also connected to the terminalT7 of the drive IC 20 via the resistor 34 a. The drain of the firstdischarge switching element 36 a is connected to the terminal T7, andthe source thereof is connected to the emitter of the switching elementS*# via a common potential line of the drive unit DU and the terminal T8of the drive IC 20.

The gate of the first discharge switching element 36 a is connected tothe drive controller 22. The gate of the switching element S*# isfurther connected to the terminal T9 of the drive IC 20 via the resistor34 b. The drain of the second discharge switching element 36 b isconnected to the terminal T9, and the source thereof is connected to theemitter of the switching element S*# via the common potential line ofthe drive unit DU and the terminal T8 of the drive IC 20. The gate ofthe second discharge switching element 36 b is connected to the drivecontroller 22.

That is, a first positive-charge dissipating path, in other words, afirst negative-charge storing path is provided between the gate of theswitching element S*# and the common potential line via the resistor 34a, the terminal T7, the first discharge switching element 36 a, and theterminal T8. Similarly, a second positive-charge dissipating path, inother words, a second negative-charge storing path is provided betweenthe gate of the switching element S*# and the common potential line viathe resistor 34 b, the terminal T9, the second discharge switchingelement 36 b and the terminal T8.

Each of the resistors 34 a and 34 b is a linear element. The resistors34 a and 34 b have respective resistances, i.e. gate resistances, Ra andRb; the resistance Ra of the resistor 34 a can be set to be equal to ordifferent from the resistance Rb of the resistor 34 b. If theresistances Ra and Rb are different from each other, the differencetherebetween results in the difference in resistance between the firstand second negative-charge storing paths. Selecting one of the first andsecond negative-charge storing paths enables active gate control for theswitching element S*#.

The switching element S*# has a sense terminal St for outputting aminute current positively associated with a current, such as a collectorcurrent, flowing through the conductive path between the collector andemitter thereof. The sense terminal St is connected to both the commonpotential line of the drive unit DU via the resistor 38 and the drivecontroller 22 via the terminal T10.

When a collector current flows through the conductive path of theswitching element S*#, a minute current positively correlated with thecollector current flows through the resistor 38, so that a voltage dropacross the resistor 38 occurs. The drive controller 22 measures thevoltage drop across the resistor 38 as a sense voltage Vse at one end ofthe resistor 38 connected to the sense terminal St. This measurementobtains, as the measured level of the sense voltage Vse, an electricstate quantity of the magnitude of the collector current flowing throughthe switching element S*#; the electric state quantity is positivelycorrelated with the magnitude of the collector current. That is, thelevel of the sense voltage Vse is as a function of, i.e. correlateswith, the magnitude of the collector current flowing through theswitching element S*#.

In addition, the gate of the switching element S*# is connected to thedrain of the off-state holding switching element 40, the source of whichis connected to the common potential line of the drive unit DU. Theoff-state holding switching element 40 is adapted to short-circuit theelectrical path between the gate and source of the switching elementS*#. In order to reduce the resistance of the gate and source of theswitching element S*# as low as possible, the off-state holdingswitching element 40 is located as close as possible to the switchingelement S*#. The impedance of an electrical path including the off-stateholding switching element 40 between the gate and source of theswitching element S*# is set to be lower than that of each of the firstand second positive-charge dissipating paths, i.e. the normal dischargepaths. The electrical path including the off-state holding switchingelement 40 will be referred to as a short-circuit path hereinafter. Thissetting aims to prevent the switching element S*# from erroneously beingON due to high-frequency noise superimposition on the gate thereofthrough the medium of parasitic capacitance between first and secondends, i.e. the emitter and collector, of the conductive path of theswitching element S*# during the OFF state of the switching element S*#.

The gate of the off-state holding switching element 40 is connected tothe drive controller 22 via the terminal T11 of the drive IC 20. Thegate of the switching element S*# is connected directly to the drivecontroller 22 via the terminal T12 of the drive IC 20. An electricalpath between the gate of the switching element S*# and the drivecontroller 22 via the terminal T12 serves as a gate-voltage monitor linefor monitoring a voltage difference between the first end (emitter) ofthe conductive path of the switching element S*# and the gate thereof asa level of gate voltage, i.e. gate-emitter voltage, Vge.

That is, the drive controller 22 is operative to monitor the level ofthe gate voltage Vge through the gate-voltage monitor line, anddetermine whether the monitored level of the gate voltage Vge is inagreement with an OFF start voltage Vgth. The drive controller 22 isoperative to turn on or off the off-state holding switching element 40using a result of the determination and the drive signal V*#.

Specifically, during the on duration of the drive signal Vi*# to chargethe gate of the switching element S*# or during the off duration of thedrive signal V*# to discharge the gate of the switching element S*# withthe monitored level of the gate voltage Vge being higher than the OFFstart voltage Vgth, the drive controller 22 turns off the off-stateholding switching element 40. On the other hand, during the off durationof the drive signal V*# to discharge the gate of the switching elementS*# with the monitored level of the gate voltage Vge being equal to orlower than the OFF start voltage Vgth, the drive controller 22 turns onthe off-state holding switching element 40.

The ON state of the off-state holding switching element 40short-circuits the gate and emitter of the switching element S*# tothereby holding the OFF state of the switching element S*#. For example,the OFF start voltage Vgth can be set to be lower than a mirror voltageof the switching element S*# described later.

On the other hand, the gate of the switching element S*# is connected tothe terminal T13 of the drive IC 20 via the resistor 42. The terminalT13 is connected to the drain of the soft-turnoff switching element 44,and the source thereof is connected to the common potential line of thedrive unit DU. The resistance Rs of the resistor 42 is set to be higherthan the resistance values Ra and Rb of the resistors 34 a and 34 b.This setting aims to increase the resistance of an electrical pathincluding the soft-turnoff switching element 44 between the gate of theswitching element S*# and the common potential line of the drive IC 20to be higher than that of each of the first and second positive-chargedissipating paths. The electrical path including the soft-turnoffswitching element 44 will be referred to as a high-resistance dischargepath hereinafter.

The comparator 46 has a non-inverting input terminal, an inverting inputterminal, and an output terminal. The non-inverting input terminal ofthe comparator 46 is connected to the terminal T10, so that the sensevoltage Vse is also inputted to the non-inverting input terminal of thecomparator 46 via the terminal T10. The inverting input terminal of thecomparator 46 is connected to a positive terminal of the reference powersource 48, and a negative terminal of the reference power source 48 isconnected to the common potential line of the drive unit DU. A referencevoltage Vref outputted from the reference power source 48 is applied tothe inverting input terminal of the comparator 46.

A level of the reference voltage Vref is set to match the level of thesense voltage Vse while the magnitude of the collector current flowingthrough the switching element S*# becomes equal to or higher than athreshold value. This setting of the reference voltage Vref causes anoutput signal of the comparator 46 to be turned from a low level, i.e. alogical OFF value, to a high level, i.e. a logical ON value, when themagnitude of the collector current becomes equal to or higher than thethreshold value. In other words, the comparator 46 is designed to outputthe signal with the low level while the magnitude of the collectorcurrent is lower than the threshold value, and output the signal withthe high level while the magnitude of the collector current is equal toor higher than the threshold value.

The delay unit 50 is configured to capture the output signal from thecomparator 46, and a fail-safe signal FL as long as the captured outputsignal from the comparator 46 is being the high level during apredetermined period. In other words, when the output signal of thecomparator 46 is turned from the low level to the high level, the delayunit 50 outputs the fail-safe signal FL after the predetermined periodhas elapsed since the change of the output signal of the comparator 46from the low level to the high level. The fail-safe signal FL is sent tothe low-voltage system via the terminal T4, and to the gate of thesoft-turnoff switching element 44. The fail-safe signal FL represents anabnormal state in the operation of the switching element S¥#. In thisembodiment, the low voltage system includes a fail-safe unit 18 b in,for example, the interface 18.

The fail-safe signal FL outputted from the delay unit 50 serves as atrigger for:

turning on the soft-turnoff switching element 44; and

instructing the drive controller 22 of each of the switching element S*#to deactivate the charge switching element 32 and the first and seconddischarge switching elements 36 a and 36 b of a corresponding one of theswitching elements S*#.

The turn-on of the soft-turnoff switching element 44 enables asoft-turnoff task of the switching element S*# to be effected, thusdissipating the charge stored in the gate of the switching element S*#via the high-resistance discharge path, so that the switching elementS*# is turned off slowly.

Because the high-resistance discharge path is higher in resistance thaneach of the first and second positive-charge dissipating paths, theturnoff speed of the switching element S*# via the high-resistancedischarge path in the abnormal state is lower than that of the switchingelement S*# via each of the first and second positive-charge dissipatingpaths in the normal state. That is, if the turnoff speed of theswitching element S*#, in other words, the rate of change of theswitching element S*# from ON to OFF, were high with the magnitude ofthe collector current exceeding the threshold value, an excessive surgemight be produced based on the turnoff of the switching element S*#.

The fail-safe unit 18 b is configured to:

receive the fail-safe signal FL; and

forcibly turn off the phototransistor of the photocouplers 18 a for therespective switching elements S*# to set the drive signals Vi*# for therespective switching elements S*#, thus turning off the respectiveswitching elements S*#.

The forcible turnoff of the respective switching elements S*# of theinverter INV and converter CNV results in shutdown of the inverter INVand converter CNV.

Next, a charging task and a discharging task for the gate of eachswitching element S*# carried out by the drive controller 22 of acorresponding drive unit DU will be described hereinafter if theswitching element S*# is operating in the normal state.

First, the charging task will be described hereinafter.

The drive controller 22 is configured to perform the charging task forthe gate of the switching element S*# using constant-current control.Specifically, the drive controller 22 controls a level of voltage to beapplied to the gate of the charge switching element S*# to regulate thevoltage drop across the resistor 30 to a target value of, for example, 1V. This regulation adjusts, to a constant level, a charge current to besupplied to the gate of the switching element S¥# from the seriesregulator 28 via the terminals T4 and T5, the resistor 30, the chargeswitching element 32, and the terminal T6. The charging task results inthe suppression of a surge produced due to the turn-on of the switchingelement S¥#. Note that the first and second discharge switching elements36 a and 36 b are OFF during the period while the charging task iscarried out.

Control of the constant-current control becomes more difficult with anincrease in the gate voltage Vge of the switching element S*#. For thisreason, in this embodiment, the output voltage Vom of the seriesregulator 28, which is actually applied to the gate of the switchingelement S*# as the gate voltage Vge, is set to be equal to or higherthan the sum of: the voltage drop across the resistor 30, the voltagedrop across the charge switching element S*#, and an upper limit of thegate voltage Vge. The upper limit of the gate voltage Vge corresponds toan upper limit of collector current as the saturation current for theswitching element S*# when the switching element S*# is normallyoperating. This setting of the output voltage Vom, i.e. the gate voltageVge, of the series regulator 28 makes it possible to prevent thecontrollability of the constant-current control from decreasing untilthe level of the gate voltage Vge reaches the upper limit of the gatevoltage Vge.

Next, the discharging task will be described hereinafter.

The drive controller 22 is configured to perform the discharging taskfor the gate of the switching element S*# using active gate control. Theactive gate control is designed to change the number of the dischargepaths, i.e. the total area of the discharging path, connected to thegate of the switching element S*# in the middle of the discharge periodfrom the start of discharging the gate of the switching element S*# tothe completion of the discharge from the gate of the switching elementS*#. That is, the active gate control increases the rate of dischargingthe gate of the switching element S*# in at least an early stage in thedischarge period, resulting in a reduction of both a surge and switchingloss when the switching element S*# is turned from ON to OFF.

For example, the drive controller 22 keeps on each of the first andsecond discharge switching elements 36 a and 36 b during an off durationof the drive signal Vi*#, thus increasing the rate of dissipatingelectrical charge stored on the gate of the switching element S*#.Thereafter, the drive controller 22 turns off any one of the first andsecond discharge switching elements 36 a and 36 b, thus reducing therate of dissipating electrical charge on the gate of the switchingelement S*#. Note that the charge switching element 32 is OFF during theperiod while the discharging task is carried out.

FIG. 3 schematically illustrates measurement results of how the gatevoltage Vge, the collector-emitter voltage Vce, the sense voltage Vse,and the collector current Ic vary after the start of discharging thegate of the switching element S*# due to the turnoff of the switchingelement S*# at time t1A. In FIG. 3, a switching element S*# isabbreviated as “SW”.

Particularly, the drive controller 22 according to this embodiment isconfigured to determine that the timing to change the rate ofdischarging the gate of the switching element S*# from a first value toa second value lower than the first value is set to predetermined timet2A. The timing to change the rate of discharging the gate of theswitching element S*# will be referred to as “discharging-rate changingtiming” hereinafter.

The time t2A is defined to time at which the collector-emitter voltageVce of the switching element S*# is substantially in agreement with theterminal voltage of the high-voltage battery 12 (see FIG. 3). The firstvalue of the discharging-rate changing timing corresponds to, forexample, the sum of the resistances Ra and Rb, and the second valuethereof corresponds to, for example, the resistance Ra or Rb. Thisdetermination of the discharging-rate changing timing delays the timingto reduce the rate of discharging the gate of the switching element S*#as much as possible, resulting in an effective reduction of a surgewhile reducing switching loss as low as possible.

In this embodiment, the drive controller 22 is configured to recognizethe time t2A as the discharging-rate changing timing as a function ofthe level of the sense voltage Vse. Specifically, the drive controller22 is configured to recognize the timing, at which the sense voltage Vsereaches a threshold voltage Vth1 after the start of the discharge fromthe gate of the switching element S*# at the time t1A, as thedischarging-rate changing timing. Using the sense voltage Vse torecognize the discharging-rate changing timing is due to the fact thatthe timing at which the collector-emitter voltage Vce is substantiallyin agreement with the output DC voltage across the capacitor C can becorrelated with the timing at which the sense voltage Vse reaches thethreshold voltage Vth1.

Specifically, even if the electromagnetic force increases up to acertain level, no excessively high voltage is applied to the switchingelement S¥# while the voltage across both ends of the conductive path ofthe switching element S¥#, i.e. the collector-emitter voltage Vce, islower than the output DC voltage across the capacitor C. Thus, thetiming at which the collector-emitter voltage Vce reaches the output DCvoltage across the capacitor C correlates with the timing at which thesense voltage Vse greatly rises to reach its peak; the timing at whichthe sense voltage Vse greatly rises is close to the time t2A at whichthe sense voltage Vse reaches the threshold voltage Vth1. Using thisrelationship between the collector-emitter voltage Vce and the sensevoltage Vse, the drive controller 22 is configured to recognize thedischarging-rate changing timing.

As described above, the sense voltage Vse rapidly rises as a spike attime close to the time t2A within the period during which the switchingelement S*# is changed from ON to OFF. The spike of the sense voltageVse is due to a surge superimposed on the sense voltage Vse viaparasitic capacitance between the emitter and collector of the switchingelement S*#.

In addition, the drive controller 22 is configured to determine whetherthe sense voltage Vse is equal to or lower than a threshold voltage Vth2set to be lower than the threshold voltage Vth1 during an on duration ofthe switching element S*#. The second threshold voltage Vth2 serves as afirst threshold value for the sense voltage Vse, and the first thresholdvoltage Vth1 serves as a second threshold value therefor.

Upon determination that the sense voltage Vse is equal to or lower thanthe threshold voltage Vth2, the drive controller 22 is configured toincrease the rate of discharging the gate of the switching element S*#without performing the active gate control, thus disabling the activegate control. The disabling of the active gate control aims to enhancethe effect of decreasing switching loss.

Specifically, the higher the collector current during an on duration ofthe switching element S*# is, the higher the magnitude of a surgeproduced due to the turnoff of the switching element S*# is. Thischaracteristic is because the higher the collector current during an onduration of the switching element S*# is, the higher the rate ofdecreasing the collector current is within the period during which theswitching element S*# is changed from ON to OFF. In view of thecharacteristic, if the collector current during an on duration of theswitching element S*# is low, the magnitude of a surge produced at theturnoff of the switching element S*# becomes low. This results in anincrease in a margin of voltage between the collector-emitter voltageVce and its acceptable upper limit. Thus, in this case of lowcollector-current, an increase in the rate of discharging the gate ofthe switching element S*# to perform the discharging task can maintain,at a high level, the reliability of the switching element S*#. In otherwords, it is possible to perform the discharging task using the activegate control by increasing the rate of discharging the gate of theswitching element S*# without effecting on the reliability of theswitching element S*#. Thus, disabling the active gate control if thecollector current during an on duration of the switching element S*# islow increases the effect of decreasing switching loss, thus improvingfuel economy of the motor vehicle.

Note that the threshold voltage Vth2 is set to be lower than thereference voltage Vref used to determine whether to perform asoft-turnoff task to discharge the gate of the switching element S*# viathe high-resistance discharge path.

In addition, FIG. 4 schematically illustrates measurement results of howthe gate voltage Vge, the collector-emitter voltage Vce, the sensevoltage Vse, and the collector current Ic vary after the start ofcharging the gate of the switching element S*# due to the turn-on of theswitching element S*# at time t3A. In FIG. 4, a switching element S*# isabbreviated as “SW”.

As illustrated in FIG. 4, as in the case of the period during which theswitching element S*# is changed from ON to OFF, the sense voltage Vserapidly rises as a spike within the period during which the switchingelement S*# is changed from OFF to ON. Such a spike is produced based onthe following reason:

If one of series-connected high-side and low-side switching elements S*pand S*n of one leg is turned on, a recovery current flows through theflywheel diode connected antiparallel to the other of the switchingelements S*p and S*n. Due to the recovery current, a surge is producedacross the flywheel diode and the other of the switching elements S*pand S*n. The surge is superimposed on the sense voltage Vse of the oneof the switching elements S*p and S*n via parasitic capacitance betweenthe emitter and collector of the one of the switching elements S*p andS*n. This superimposition results in a spike with the period duringwhich the switching element S*# is changed from OFF to ON.

The occurrence of a spike during an on state of the switching elementS*# may cause various malfunctions in the drive unit DU. As one exampleof these malfunctions, the sense voltage Vse may exceed the thresholdvoltage Vth2 due to a spike although the collector current issufficiently low. In this case, the sense voltage Vse exceeding thethreshold voltage Vth2 may cause the drive controller 22 to perform theactive gate control by mistake although there is room to disable theactive gate control to increase the switching speed in the dischargingtask. This may result in a reduction of the effect of decreasingswitching loss.

As another example of these malfunctions, the sense voltage Vse mayexceed the threshold voltage Vth1 due to a spike although the chargingtask is performed to change the switching element S*# from OFF to ON,resulting in the discharging task based on the active gate control beingperformed erroneously.

In view of these circumstances, the drive controller 22 of each driveunit DU is configured to perform a charging and discharging routine forthe gate of the switching element S*# during an on duration of the drivesignal g*# in accordance with the following procedure illustrated inFIG. 5. Note that the drive controller 22 of each drive unit DU can beconfigured as a programmed logic circuit, a hard-wired logic circuit, orthe combination of hardwired-logic and programmed-logic hybrid circuits.The drive controller 22 is configured to repeatedly carry out thecharging and discharging routine each time the drive signal g*# isturned from OFF to ON, in other words, each time the drive signal Vi*#is turned from the high level to the low level.

Because the voltage drop across the resistor 38 is constantly inputtedto the drive controller 22, the drive controller 22 constantly measuresthe voltage drop across the resistor 38 as a level of a sense voltageVse in step S4.

When the drive signal Vi*# inputted to the drive controller 22 is turnedfrom the off command, i.e. the high level, to the on command, i.e. thelow level, the drive controller 22 performs the charging task to turn onthe charge switching element 32 while turning off at least one of thefirst and second discharge switching elements 36 a and 36 b, which havebeen ON in step S6. In step S6, the drive controller 22 also starts tomeasure time elapsed since the change of the drive signal Vi*# from theoff command to the on command in step S6.

In addition, in response to the change of the drive signal Vi*# from theoff command to the on command, the drive controller 22 invalidates themeasured level of the sense voltage Vse to be used for comparison withthe threshold voltage Vth1, which has been validated in the previousdischarging task described later in step S8. The operation in step S8disables the measured level of the sense voltage Vse from being used todetermine the discharging-rate changing timing.

Next, in step S10, the drive controller 22 determines whether a presetfirst threshold time TA has elapsed since the change of the drive signalVi*# from the off command to the on command.

That is, the operation in step S10 is required for the drive controller22 to wait until a spike, which may be produced due to the turn-on ofthe drive signal g*#, is sufficiently suppressed. In other words, thefirst threshold time TA is required for a spike, which may be produceddue to the turn-on of the drive signal g*#, to be sufficientlysuppressed. That is, the first threshold time TA is required to make themeasured value of the sense voltage Vse stable. This is because,immediately after the drive signal Vi*# is changed to the on state, themeasured value of the sense voltage Vse may be unstable due to, forexample, such a surge, noise, and the instability of the collectorcurrent flowing through the switching element S*#.

Specifically, upon determination that the first threshold time TA hasnot elapsed since the change of the drive signal Vi*# from the offcommand to the on command (NO in step S10), the drive controller 22waits to carry out the operation in step S12. Otherwise, upondetermination that the first threshold time TA has elapsed since thechange of the drive signal Vi*# from the off command to the on command(YES in step S10), the drive controller 22 carries out the operation instep S12.

In step S12, the drive controller 22 validates the measured level of thesense voltage Vse to be used for comparison with the threshold voltageVth2. The operation in step S12 enables the measured level of the sensevoltage Vse to be used to determine whether to perform a task ofdisabling the active gate control.

Next, the drive controller 22 determines whether the measured level ofthe sense voltage Vse becomes equal to or higher than the thresholdvoltage Vth2 until the time when the drive signal Vi*# is changed fromthe on command to the off command in steps S14, S16, and S18. Theseoperations in steps S14, S16, and S18 aim to determine whether toperform the task of disabling the active gate control.

Specifically, the drive controller 22 determines whether the measuredlevel of the sense voltage Vse becomes equal to or higher than thethreshold voltage Vth2 in step S14.

At that time, because the operation in step S14 is carried out after thelapse of the first threshold time TA since the change of the drivesignal Vi*# to the on command, it is accurately recognize therelationship between the measured level of the sense voltage Vse and thethreshold voltage Vth2. This results in an accurate recognition of acertain margin of voltage between the collector-emitter voltage Vse andan acceptable upper limit therefor as a function of the accuratelyrecognized relationship between the measured level of the sense voltageVse and the threshold voltage Vth2.

Upon determination that the measured level of the sense voltage Vsebecomes equal to or higher than the threshold voltage Vth2 (YES in stepS14), the drive controller 22 sets a disabling flag F as an identifierto 1 indicative of non-execution of the task of disabling the activegate control in step S16. The disabling flag F, for example, is in theform of a bit (i.e. 0 or 1). The disabling flag F of 0 is indicative ofexecution of the task of disabling the active gate control. An initialvalue of the disabling flag F is set to 0. After the operation in stepS16, the drive controller 22 carries out the operation in step S18.

Otherwise, upon determination that the measured level of the sensevoltage Vse is lower than the threshold voltage Vth2 (NO in step S14),the drive controller 22 carries out the operation in step S18 whileskipping the operation in step S16, thus keeping the disabling flag F at0.

In step S18, the drive controller 22 determines whether the drive signalVi*# is changed from the on command to the off command. Upondetermination that the drive signal Vi*# is not changed from the oncommand to the off command (NO in step S18), the drive controller 22repeats the operations in steps S14, S16, and S18. As a result, upondetermination that the drive signal Vi*# is changed from the on commandto the off command (YES in step S18), the drive controller 22 carriesout the operation in step S20.

In step S20, the drive controller 22 invalidates the measured level ofthe sense voltage Vse to be used for comparison with the thresholdvoltage Vth2, and validates the measured level of the sense voltage Vseto be used for comparison with the threshold voltage Vth1. The operationin step S20 disables the measured level of the sense voltage Vse to beused to determine whether to perform a task of disabling the active gatecontrol, and enables the measured level of the sense voltage Vse to beused to determine the discharging-rate changing timing.

Next, the drive controller 22 determines whether the disabling flag F isset to 1 in step S22.

Upon determination that the disabling flag F is set to 1 (YES in stepS22), the drive controller 22 determines not to perform the task ofdisabling the active gate control, and therefore carries out thedischarging task using the operation in one of steps S24, S26, and S28.

Specifically, the drive controller 22 determines whether the measuredlevel of the sense voltage Vse is lower than the threshold voltage Vth1in step S24. Upon determination that the measured level of the sensevoltage Vse is lower than the threshold voltage Vth1 (YES in step S24),the drive controller 22 carries out the operation in step S26. In stepS26, the drive controller 22 turns on both the first and seconddischarge switching elements 36 a and 36 b and keeps them on in step S26until the measured level of the sense voltage Vse becomes equal to orhigher than the threshold voltage Vth1 (NO in step S24). That is, theoperation to keep on the first and second discharge switching elements36 a and 36 b in step S26 increases the rate of discharging the gate ofthe switching element S*#.

Upon determination that the measured level of the sense voltage Vsebecomes equal to or higher than the threshold voltage Vth1 (NO in stepS24), the drive controller 22 turns off any one of the first and seconddischarge switching elements 36 a and 36 b and keeps off it based on theactive gate control in step S28. The operation to keep off any one ofthe first and second discharge switching elements 36 a and 36 b in stepS28 decreases the rate of discharging the gate of the switching elementS*#.

That is, the first positive-charge dissipating path including the firstdischarge switching element 36 a and the resistor 34 a, and the secondpositive-charge dissipating path including the second dischargeswitching element 36 b and the resistor 34 b serve as, for example, adischarging-rate changing module adapted to change the rate ofdischarging the gate of the switching element S*#.

The operation in step S4 by the drive controller 22 serve as, forexample, a measuring module configured to measure a value of a parameteras a function of the collector current flowing through the conductivepath of the switching element S*#.

The operations in steps S10, S12, S14, S16, S18, S20, S22, S24, S26, andS28 by the drive controller 22 serve as, for example, a dischargingcontrol module configured to:

control the discharging-rate changing module as a function of the valueof the parameter to select one of the first value and the second valueas the rate of discharging the gate of the switching element S*# uponthe drive signal Vi*# directing a change from the on command to the offcommand; and

discharge the gate of the switching element S*# using the selected oneof the first value and the second value as the rate of discharging thegate of the switching element S*#.

Otherwise, upon determination that the disabling flag F is set to 0 (NOin step S22), the drive controller 22 determines to perform the task ofdisabling the active gate control, in other words, determines not toperform the active gate control. Then, the drive controller 22 turns onboth the first and second discharge switching elements 36 a and 36 b andkeeps them on in step S32.

After one of the operations in steps S26, S28, and S32, the drivecontroller 22 determines whether the drive signal V*# is changed fromthe off command to the on command in step S30.

That is, the drive IC 22 repeatedly performs one of these operations instep S26, S28, and S32 as long as the drive signal V*# is not changedfrom the off command to the on command (NO in step S30), in other words,until the drive signal V*# is changed from the off command to the oncommand.

Upon determination that the drive signal Vi*# is changed from the offcommand to the on command (YES in step S30), the routine returns to stepS6, and the drive controller 22 carries out the operations in steps S6to S30 set forth above.

Note that a spike produced during the period when the switching elementS*# is turned from OFF to ON or from ON to OFF may cause thesoft-turnoff switching element 44 to be turned on, so that the gate ofthe switching element S*# may be discharged via the high-resistancedischarge path erroneously. This may slow the turnoff speed of theswitching element S*# by mistake.

Thus, the drive controller 22 according to this embodiment performs asoft-turnoff disabling task to invalidate the measured level of thesense voltage Vse to be used to determine whether to perform thesoft-turnoff task.

Specifically, in step S12, the drive controller 22 validates themeasured level of the sense voltage Vse to be used for comparison withthe reference voltage Vref. The operation in step S12 enables themeasured level of the sense voltage Vse to be used to determine whetherto perform the soft-turnoff task.

In addition, in step S20, the drive controller 22 invalidates themeasured level of the sense voltage Vse to be used for comparison withthe reference voltage Vref. The operation in step S12 enables themeasured level of the sense voltage Vse to be used to determine whetherto perform the soft-turnoff task.

In other words, the drive controller 22 disables the measured level ofthe sense voltage Vse to be used to determine whether to perform thesoft-turnoff task within the first period until the first threshold timeTA has elapsed since the change of the drive signal Vi*# from the offcommand to the on command and also within the second period during theon duration of the drive signal Vi*#.

The soft-turnoff disabling task prevents the soft-turnoff task frombeing carried out erroneously.

FIGS. 6 and 7 schematically illustrate specific operations of variablescorrelated with the charging and discharging routine illustrated in FIG.5.

Particularly, FIG. 6 schematically illustrates specific operations ofthe variables correlated with the charging and discharging routine usingthe active gate control. In FIGS. 6 and 7, active gate control isabbreviated as “AGC”.

Let us describe the specific operations of the variables correlated withthe charging and discharging routine using the active gate control withreference to FIG. 6.

How the drive signal g*# varies is illustrated in (a) of FIG. 6, and howthe drive signal Vi*# varies is illustrated in (b) of FIG. 6. How thegate voltage Vge varies is illustrated in (c) of FIG. 6, and how thesense voltage Vse varies is illustrated in (d) of FIG. 6. How the rateof dissipating electrical charge stored on the gate of the switchingelement S*# as the rate of discharging the gate of the switching elementS*# is illustrated in (e) of FIG. 6. How the sense voltage Vse to beused for comparison with the threshold voltage Vth1 varies as “valid” or“invalid” is illustrated in (f) of FIG. 6, and how the sense voltage Vseto be used for comparison with the threshold voltage Vth2 varies as“valid” or “invalid” is illustrated in (g) of FIG. 6. How the sensevoltage Vse to be used for comparison with the reference voltage Vrefvaries as “valid” or “invalid” is illustrated in (h) of FIG. 6.

When a drive signal Vi*# inputted to the drive unit DU is changed fromthe off command to the on command at time t1, the charging task for thegate of the switching element S*# is started at the time t1. Thischarging task results in the gate of the switching element S*# startingto rise from time t2 (see steps S6 and S8). Note that the reason why ittakes a certain period before the gate voltage Vge starts to rise inresponse to the change of the drive signal Vi*# from the off command tothe on command is signal delay in the drive unit DU.

Note that, as illustrated in (c) FIG. 6, during the on command of thedrive signal Vi*#, the gate voltage Vge increases at a predeterminedfirst gradient first. Then, the gate voltage Vge stays at a middlepotential between 0 V and the output voltage level Vom of the seriesregulator 28.

Thereafter, the gate voltage Vge increases at a predetermined secondgradient again. The period during which the gate voltage Vge has stayedat the middle potential will be referred to as a mirror periodhereinafter. The mirror period is defined depending on thecharacteristics of a corresponding switching element S*#.

After the start of the increase in the gate voltage Vge, the measuredlevel of the sense voltage Vse to be used for comparison with thethreshold voltage Vth2 is validated at the time (see t3) when the firstthreshold time TA has elapsed since the change of the drive signal Vi*#from the off command to the on command (see step S12). Thereafter, whenit is determined that the measured level of the sense voltage Vse isequal to or higher than the threshold voltage Vth2 at time t4 within theperiod during which the measured level of the sense voltage Vse to beused for comparison with the threshold voltage Vth2 is validated (seeYES in step S14), the disabling flag F is set to 1 (see step S16). Thedisabling flag F being set to 1 disables the drive controller 22 fromexecuting the task of disabling the active gate control. In other words,the disabling flag F being set to 1 enables the drive controller 22 toexecute the active gate control.

Thereafter, when the drive signal Vi*# is changed from the on command tothe off command at time t5 (see YES in step S18), the measured level ofthe sense voltage Vse to be used for comparison with the thresholdvoltage Vth2 is invalidated, and the measured level of the sense voltageVse to be used for comparison with the threshold voltage Vth1 isvalidated (see step S20).

At that time, because the disabling flag F is 1 so that thedetermination in step S22 is YES, the active gate control is carried outin steps S26 and S28. Specifically, while the measured level of thesense voltage Vse is lower than the threshold voltage Vth1 (see YES instep S24), the gate of the switching element S*# is discharged via boththe first and second positive-charge dissipating paths, so that adecrease in the gate voltage Vge is started at time t6. At that time,the rate of discharging the gate of the switching element S*# is set toa higher value (see step S26 and the period from the time t5 to time t7in FIG. 6).

Note that, as in the case of the increase in the gate voltage Vge,during the off command of the drive signal Vi*#, the gate voltage Vgedecreases at a predetermined first gradient first. Then, the gatevoltage Vge stays at the middle potential during the mirror period, andthereafter decreases at a predetermined second gradient again.

When the measured level of the sense voltage Vse reaches the thresholdvoltage Vth1 (see NO in step S24), the active gate control is carriedout, so that the gate of the switching element S*# is discharged via oneof the first and second positive-charge dissipating paths. This changesthe rate of discharging the gate of the switching element S*# from thehigher value to a lower value (see step S28 and the time t7 in FIG. 6).

Thereafter, when the drive signal Vi*# is changed from the off commandto the on command at time t8, the measured level of the sense voltageVse to be used for comparison with the threshold voltage Vth1 isinvalidated (see step S8).

In addition, the soft-turnoff task is performed within the first perioduntil the first threshold time TA has elapsed since the change of thedrive signal Vi*# from the off command to the on command (see the periodbefore the time t1), and the second period during the on duration of thedrive signal Vi*# (see the period from the time t5 to the time t8). Thesoft-turnoff disabling task prevents the soft-turnoff task from beingcarried out even if the level of the sense voltage Vge exceeds thereference voltage Vref due to a surge produced when the switchingelement S*# is changed from ON to OFF or OFF to ON.

Next, let us describe the specific operations of the variablescorrelated with the charging and discharging routine while the activegate control is not used with reference to FIG. 7. (a) to (g) of FIG. 7correspond to (a) to (g) of FIG. 6. Note that, in FIG. 7, theillustration of how the sense voltage Vse to be used for comparison withthe reference voltage Vref varies as “valid” or “invalid” is omittedbecause it is identical to that in (h) of FIG. 6.

When a drive signal Vi*# inputted to the drive unit DU is changed fromthe off command to the on command at time t11, the charging task for thegate of the switching element S*# is started at the time t11 so that thegate of the switching element S*# starts to rise (see steps S6 and S8).

Thereafter, the measured level of the sense voltage Vse to be used forcomparison with the threshold voltage Vth2 is validated at the time (seet12) when the first threshold time TA has elapsed since the change ofthe drive signal Vi*# from the off command to the on command (see YES instep S10 and step S12).

Thereafter, it is determined that the measured level of the sensevoltage Vse is kept to be lower than the threshold voltage Vth2 withinthe period from the time t12 to time t13 during which the measured levelof the sense voltage Vse to be used for comparison with the thresholdvoltage Vth2 is validated (see NO in step S14). This results in thedisabling flag F being kept to 0 (see the skip of the operation in stepS16). The disabling flag F being set to 0 enables the drive controller22 to execute the task of disabling the active gate control, in otherwords, disables the drive controller 22 from executing the active gatecontrol.

Thereafter, when the drive signal Vi*# is changed from the on command tothe off command at the time t13 (see YES in step S18), the measuredlevel of the sense voltage Vse to be used for comparison with thethreshold voltage Vth2 is invalidated, and the measured level of thesense voltage Vse to be used for comparison with the threshold voltageVth1 is validated (see step S20).

At that time, because the disabling flag is 0 so that the determinationin step S22 is NO, the active gate control is not carried out.Specifically, the gate of the switching element S*# is discharged viaboth the first and second positive-charge dissipating paths during theperiod from the time t13 to time t14, so that the rate of dischargingthe gate of the switching element S*# is kept to the higher value (seestep S32).

Thereafter, when the drive signal Vi*# is changed from the off commandto the on command at the time t14, the measured level of the sensevoltage Vse to be used for comparison with the threshold voltage Vth1 isinvalidated (see step S8).

Next, technical effects achieved by the charging and discharging routinefor the gate of the switching element S*# during an on duration of thedrive signal g*# according to this embodiment will be describedhereinafter with reference to FIG. 8.

FIG. 8 schematically illustrates a first measured correlation betweencollector current Ic and surge voltage for each switching element S*#when the charging and discharging routine according to this embodimentis applied to the switching element S*# (see thick solid line in theupper side of FIG. 8). FIG. 8 schematically illustrates a secondmeasured correlation between collector current Ic and surge voltage foreach switching element S*# when a known charging and discharging routineusing a positive-charge dissipating path having a combined resistance,i.e. a fixed resistance, of the resistors 34 a and 34 b connected inparallel (see dashed line in the upper side of FIG. 8).

In addition, FIG. 8 schematically illustrates a third measuredcorrelation between collector current Ic and switching loss for eachswitching element S*# when the charging and discharging routineaccording to this embodiment is applied to the switching element S*#(see thick solid line in the lower side of FIG. 8). FIG. 8 schematicallyillustrates a fourth measured correlation between collector current Icand switching loss for each switching element S*# when the knowncharging and discharging routine using the positive-charge dissipatingpath having the combined resistance of the resistors 34 a and 34 bconnected in parallel (see dashed line in the lower side of FIG. 8).

FIG. 8 demonstrates that switching loss of each switching element S*#resulted from the charging and discharging routine according to thisembodiment is reduced in comparison to that of each switching elementS*# resulted from the known charging and discharging routine using thepositive-charge dissipating path having the combined resistance of theresistors 34 a and 34 b connected in parallel. Note that the reason whyswitching loss within the period during which the task of disabling theactive gate control (AGC), resulted from the charging and dischargingroutine according to this embodiment, is reduced is that the task ofdisabling the active gate control increases the switching speed of theswitching element S*#.

As described above, the drive unit DU for each switching element S*#according to this embodiment is configured to perform the task ofdisabling the active gate control when determining that a predeterminedcondition is met. The predetermined condition is that:

the first threshold time TA has elapsed since the change of the drivesignal Vi*# from the off command to the on command; and

the measured level of the sense voltage Vse is kept to be lower than thesecond threshold Vth2 within the period during which the drive signalVi*# directs the on command.

This configuration enables the measured level of the sense voltage Vseto be used for determination of whether to perform the task of disablingthe active gate control independently of a spike that may be produceddue to the turn-on of the drive signal g*#.

Thus, it is possible to maintain, at a high level, the effect ofreducing switching loss independently of a spike that may be produceddue to the turn-on of the drive signal g*#, thus improving fuel economyof the motor vehicle in which the drive unit DU for each switchingelement S*# is installed.

The drive unit DU for each switching element S*# is configured to definea validated period for the measured level of the sense voltage Vse to beused for comparison with the threshold voltage Vth2. The validatedperiod is the period during which the drive signal Vi*# directs the oncommand after the lapse of the first threshold time TA since the changeof the drive signal Vi*# from the off command to the on command. Thisconfiguration enables the drive controller 22 to reliably determinewhether there is a need to perform the task of disabling the active gatecontrol.

The drive unit DU for each switching element S*# is configured tovalidate the measured level of the sense voltage Vse to be used forrecognition of the discharging-rate changing timing within only theperiod during which the drive signal Vi*# directs the off command. Thisconfiguration prevents the discharging task from being carried outerroneously although the charging task for the gate of the switchingelement S*# is instructed by the drive signal g*#.

The drive unit DU for each switching element S*# is configured torecognize the discharging-rate changing timing as a function of thesense voltage Vse. This configuration eliminates the need to provide acircuit structure installed in the low voltage system and designed tosupport the drive unit DU to recognize the discharging-rate changingtiming. Thus, it is possible to prevent the scale of the drive unit DUfrom expanding into the low voltage system.

The drive unit DU for each switching element S*# is configured toperform the soft-turnoff disabling task of the switching element S*# tothereby prevent slow turnoff of the switching element S*#. Thisconfiguration reliably prevents the soft-turnoff disabling task frombeing carried out by mistake due to a spike produced during the changeof the drive voltage g*# from one of the on command and the off commandto the other thereof.

Second Embodiment

A control system for controlling the motor-generator 10 according to asecond embodiment of the present disclosure will be described withreference to FIGS. 9A and 9B.

The structure and/or functions of the control system according to thesecond embodiment are mainly identical to those of the control systemaccording to the first embodiment except for the following points. So,the different points mainly will be described hereinafter.

The charging and discharging routine for each switching element S*#according to the second embodiment is configured to validate themeasured level of the sense voltage Vse to be used for comparison withthe threshold voltage Vth2 as long as:

the drive signal Vi*# directs the on command; and

the measured level of the sense voltage Vse has been equal to or higherthan a preset voltage level Vα.

The voltage level Vα is previously set to a voltage level correspondingto the timing at which a spike produced based on the change of the drivesignal Vi*# from the off command to the on command is sufficientlysuppressed. For example, the voltage level can be set to be equal to orhigher than a predetermined threshold voltage at which the switchingelement S*# has been completely ON; the predetermined threshold voltageis the mirror voltage or thereabout.

FIG. 9A schematically illustrates the specific operations of thevariables correlated with the charging and discharging routine while theactive gate control is not used according to this embodiment. (a) to (e)of FIG. 9A correspond to (a) to (d), and (g) of FIG. 6. Note that, inFIG. 9A, the illustration of how the rate of discharging the gate of theswitching element S*# varies and that of how the sense voltage Vse to beused for comparison with the threshold voltage Vth1 varies as “valid” or“invalid” are omitted because they are identical to those in (e) and (f)of FIG. 6.

When a drive signal Vi*# inputted to the drive unit DU is changed fromthe of command to the on command at time t21, the charging task for thegate of the switching element S*# is started at the time t21 so that thegate of the switching element S*# starts to rise (see steps S6 and S8).

Thereafter, the measured level of the sense voltage Vse to be used forcomparison with the threshold voltage Vth2 is validated at the time (seet22) when the measured level of the sense voltage Vse has been equal toor higher than the preset voltage level Vα (see YES in step S10A of FIG.9B and step S12). The operation in step S10A by the drive controller 22serves as, for example, the discharging control module.

Thereafter, it is determined that the measured level of the sensevoltage Vse is kept to be lower than the threshold voltage Vth2 withinthe period from the time t12 to time t23 during which the measured levelof the sense voltage Vse to be used for comparison with the thresholdvoltage Vth2 is validated (see NO in step S14). This results in thedisabling flag F being kept to 0 (see the skip of the operation in stepS16). The disabling flag F being set to 0 enables the drive controller22 to execute the task of disabling the active gate control.

Thereafter, when the drive signal Vi*# is changed from the on command tothe off command at the time t23 (see YES in step S18), the measuredlevel of the sense voltage Vse to be used for comparison with thethreshold voltage Vth2 is invalidated, and the measured level of thesense voltage Vse to be used for comparison with the threshold voltageVth1 is validated (see step S20).

Other operations of the variables correlated with the charging anddischarging routine according to this embodiment are substantiallyidentical to corresponding operations of the variables correlated withthe charging and discharging routine according to the first embodiment.

Specifically, the drive unit DU for each switching element S*# accordingto this embodiment is configured to perform the task of disabling theactive gate control when determining that a predetermined condition ismet. The predetermined condition is that the measured level of the sensevoltage Vse has been equal to or higher than the preset voltage levelVα, and the drive signal Vi*# directs the on command. The voltage levelVα is previously set to a voltage level corresponding to the timing atwhich a spike produced based on the change of the drive signal Vi*# fromthe off command to the on command is sufficiently suppressed.

This configuration enables the measured level of the sense voltage Vseto be used for determination of whether to perform the task of disablingthe active gate control independently of a spike that may be produceddue to the turn-on of the drive signal g*#. Thus, the configuration ofeach drive unit DU according to the second embodiment achieves the sameeffects as the configuration of each drive unit DU according to thefirst embodiment.

Third Embodiment

A control system for controlling the motor-generator 10 according to athird embodiment of the present disclosure will be described withreference to FIGS. 10A and 10B.

The structure and/or functions of the control system according to thesecond embodiment are mainly identical to those of the control systemaccording to the first embodiment except for the following points. So,the different points mainly will be described hereinafter.

The charging and discharging routine for each switching element S*#according to the third embodiment is configured to validate the measuredlevel of the sense voltage Vse to be used for comparison with thethreshold voltage Vth2 if it is determined that the measured level ofthe sense voltage Vse at the timing when the drive signal Vi*# ischanged from the on command to the off command is lower than thethreshold voltage Vth2.

FIG. 10A schematically illustrates the specific operations of thevariables correlated with the charging and discharging routine while theactive gate control is not used according to this embodiment. (a) to (e)of FIG. 10 correspond to (a) to (d), and (g) of FIG. 6. Note that, inFIG. 10A, the illustration of how the rate of discharging the gate ofthe switching element S*# varies and that of how the sense voltage Vseto be used for comparison with the threshold voltage Vth1 varies as“valid” or “invalid” are omitted because they are identical to those in(e) and (f) of FIG. 6.

When a drive signal Vi*# inputted to the drive unit DU is changed fromthe off command to the on command at time t31, the charging task for thegate of the switching element S*# is started at the time t31 so that thegate of the switching element S*# starts to rise (see steps S6 and S8 ofFIG. 10B).

After the operation in step S8, the routine proceeds to step S13 of FIG.10B. Specifically, the drive controller 22 determines whether the drivesignal Vi*# is changed from the low level to the high level in step S13

Upon determination that that the drive signal Vi*# is not changed fromthe low level to the high level (NO in step S13), the drive controller22 repeats the operation in step S13. As a result, upon determinationthat the drive signal Vi*# is changed from the low level to the highlevel (YES in step S13), the drive controller 22 carries out theoperation in step S14 a of FIG. 10B.

Specifically, the drive controller 22 validates the measured level ofthe sense voltage Vse to be used for comparison with the thresholdvoltage Vth2 in step S14 a (see time t32). Then, the drive controller 22determines whether the measured level of the sense voltage Vse becomesequal to or higher than the threshold voltage Vth2 at the timing whenthe drive signal Vi*# is changed from the on command to the off command(see step S14 b and the time t32).

Upon determination that the measured level of the sense voltage Vse isequal to or higher than the threshold voltage Vth2 at the timing whenthe drive signal Vi*# is changed from the on command to the off command(YES in step S14 b and the solid line indicative of transition of thesense voltage Vse at the time t32)), the drive controller 22 sets thedisabling flag F to 1 indicative of non-execution of the task ofdisabling the active gate control in step S16. Thereafter, the routineproceeds to step S20 set forth above. That is, the active gate controlis carried out after the time t32.

Otherwise, upon determination that the measured level of the sensevoltage Vse is lower than the threshold voltage Vth2 at the timing whenthe drive signal Vi*# is changed from the on command to the off command(NO in step S14 b and the dashed line indicative of transition of thesense voltage Vse at the time t32), the drive controller 22 carries outthe operation in step S20 while skipping the operation in step S16, thuskeeping the disabling flag F at 0.

Thus, the active gate control is disabled after the time t32.

The operations in steps S13, 14 a, and 14 b by the drive controller 22serve as, for example, the discharging control module.

Other operations of the variables correlated with the charging anddischarging routine according to this embodiment are substantiallyidentical to corresponding operations of the variables correlated withthe charging and discharging routine according to the first embodiment.

Specifically, the drive unit DU for each switching element S*# accordingto this embodiment is configured to validate the measured level of thesense voltage Vse at the timing t32 when the drive signal Vi*# ischanged from the on command to the off command.

This configuration enables the measured level of the sense voltage Vseto be used for determination of whether to perform the task of disablingthe active gate control independently of a spike that may be produceddue to the turn-on of the drive signal g*#. Thus, the configuration ofeach drive unit DU according to the second embodiment achieves the sameeffects as the configuration of each drive unit DU according to thefirst embodiment.

In addition, the configuration prevents the switching element S*# fromdriven erroneously due to any cause except for a spike occurring withinthe period from the time t31 to the time t32 illustrated in FIG. 10A.

That is, during the drive signal Vi*# directing the on command so thatthe switching element S*# is ON, the measured value of the sense voltageVse may be unstable due to, for example, noise and a gradual increase inthe collector current flowing through the switching element S*#. In viewof this point, the configuration of the drive unit DU according to thisembodiment does not give the drive controller 22 any chance to use themeasured value of the sense voltage Vse at the timing immediately beforethe drive signal Vi*# becoming the off command for comparison with thethreshold voltage Vth2. In addition, the configuration of the drive unitDU according to this embodiment prevents noise from entering themeasured value of the sense voltage Vse during the switching element S*#being ON.

Accordingly, the relationship between the measured level of the sensevoltage Vse and the threshold voltage Vth2 is actually recognized. Thisresults in an accurate recognition of a certain margin of voltagebetween the collector-emitter voltage Vse and an acceptable upper limittherefor as a function of the accurately recognized relationship betweenthe measured level of the sense voltage Vse and the threshold voltageVth2.

Fourth Embodiment

A control system for controlling the motor-generator 10 according to afourth embodiment of the present disclosure will be described withreference to FIGS. 11A and 11B.

The structure and/or functions of the control system according to thefourth embodiment are mainly identical to those of the control systemaccording to the first embodiment except for the following points. So,the different points mainly will be described hereinafter.

The charging and discharging routine for each switching element S*#according to the fourth embodiment is configured to validate themeasured level of the sense voltage Vse to be used for comparison withthe threshold voltage Vth2 as long as:

the drive signal Vi*# directs the on command;

it is determined that the measured level of the sense voltage Vse isequal to or higher than the threshold voltage Vth2 at a timing as areference timing; and

a preset second threshold time TB has elapsed since the reference timingwhile the measured level of the sense voltage Vse is kept to be equal toor higher than the threshold voltage Vth2.

FIG. 11A schematically illustrates the specific operations of thevariables correlated with the charging and discharging routine while theactive gate control is not used according to this embodiment. (a) to (e)of FIG. 11A correspond to (a) to (d), and (g) of FIG. 6. Note that, inFIG. 11A, the illustration of how the rate of discharging the gate ofthe switching element S*# varies and that of how the sense voltage Vseto be used for comparison with the threshold voltage Vth1 varies as“valid” or “invalid” are omitted because they are identical to those in(e) and (f) of FIG. 6.

When a drive signal Vi*# inputted to the drive unit DU is changed fromthe off command to the on command at time t41, the charging task for thegate of the switching element S*# is started at the time t41 so that thegate of the switching element S*# starts to rise (see steps S6 and S8).

Thereafter, the drive controller 22 determines whether the measuredlevel of the sense voltage Vse becomes equal to or higher than thethreshold voltage Vth2 in step S50 of FIG. 11B.

Upon determination that the measured level of the sense voltage Vse islower than the threshold voltage Vth2 (NO in step S50), the drivecontroller 22 determines whether the drive signal Vi*# is changed fromthe on command to the off command in step S18 a. Upon determination thatthe drive signal Vi*# is not changed from the on command to the offcommand (NO in step S18 a), the drive controller 22 repeats theoperation in step S50. As a result, upon determination that the drivesignal Vi*# is changed from the on command to the off command (YES instep S18), the routine proceeds to step S20 with the disabling flag Fbeing kept to 0.

Otherwise, upon determination that the measured level of the sensevoltage Vse becomes equal to or higher than the threshold voltage Vth2at time t42 (YES in step S50), the drive controller 22 holds the timet42 as a reference timing, and carries out the operation in step S52.

Specifically, the drive controller 22 determines whether the presetsecond threshold time TB has elapsed since the reference timing t42while the measured level of the sense voltage Vse is kept to be equal toor higher than the threshold voltage Vth2 in step S52.

Upon determination that the measured level of the sense voltage Vsebecomes lower than the threshold voltage Vth2 until the second thresholdtime has elapsed since the reference timing t42 (NO in step S52), thedrive controller 22 carries out the operation in step S18 a.

Otherwise, upon determination that the preset second threshold time TBhas elapsed at time t43 since the reference timing t42 while themeasured level of the sense voltage Vse is kept to be equal to or higherthan the threshold voltage Vth2 (YES in step S52), the drive controller22 carries out the operation in step S12. In step S12, the drivecontroller 22 validates the measured level of the sense voltage Vse tobe used for comparison with the reference voltage Vref, and sets thedisabling flag F to 1 indicative of non-execution of the task ofdisabling the active gate control in step S16 (see the time t43).

Thereafter, the routine proceeds to step S20 in response to the timewhen the drive signal Vi*# is changed from the on command to the offcommand (see YES in step S18 b).

In step S20, as described above, the drive controller 22 invalidates themeasured level of the sense voltage Vse to be used for comparison withthe threshold voltage Vth2, and validates the measured level of thesense voltage Vse to be used for comparison with the threshold voltageVth1 (see time t44).

The operations in steps S50 and S52 by the drive controller 22 serve as,for example, the discharging control module.

Other operations of the variables correlated with the charging anddischarging routine according to this embodiment are substantiallyidentical to corresponding operations of the variables correlated withthe charging and discharging routine according to the first embodiment.

Specifically, the drive unit DU for each switching element S*# accordingto this embodiment is configured to perform the task of disabling theactive gate control when determining that a predetermined condition ismet. The predetermined condition is that:

the drive signal Vi*# directs the on command;

it is determined that the measured level of the sense voltage Vse isequal to or higher than the threshold voltage Vth2 at a timing as thereference timing; and

the preset second threshold time TB has elapsed since the referencetiming while the measured level of the sense voltage Vse is kept to beequal to or higher than the threshold voltage Vth2.

This configuration enables the measured level of the sense voltage Vseto be used for determination of whether to perform the task of disablingthe active gate control independently of a spike that may be produceddue to the turn-on of the drive signal g*#. Thus, the configuration ofeach drive unit DU according to the second embodiment achieves the sameeffects as the configuration of each drive unit DU according to thefirst embodiment.

In addition, the configuration prevents the switching element S*# fromdriven erroneously even if noise temporarily enters the measured levelof the sense voltage Vse within the period from the time t41 to the timet44. This is because, even if the measured level of the sense voltageVse temporarily exceeded the threshold voltage Vth2 due to suchtemporary noise, the measured level of the sense voltage Vse couldbecome lower than the threshold voltage Vth2 before the preset thresholdtime TB has elapsed since the corresponding reference timing.

More specifically, if the drive controller 22 used the measured level ofthe sense voltage Vse for comparison with the second threshold Vth2without the lapse of the threshold time TB since the reference timing,the rate of discharging the gate of the switching element S*# might bedetermined erroneously, resulting in reduction of the effect ofdecreasing switching loss.

However, the configuration of the drive unit DU according to thisembodiment prevents the rate of discharging the gate of the switchingelement S*# from being determined erroneously.

Fifth Embodiment

A control system for controlling the motor-generator 10 according to afifth embodiment of the present disclosure will be described withreference to FIGS. 12, 13, and 14.

The structure and/or functions of the control system according to thefifth embodiment are mainly identical to those of the control systemaccording to the first embodiment except for the following points. So,the different points mainly will be described hereinafter.

The charging and discharging routine for each switching element S*#according to the first embodiment is configured to change the rate ofdischarging the gate of the switching element S*# in the middle of thedischarge period from the start of discharging the gate of the switchingelement S*# to the completion of the discharge from the gate of theswitching element S*#.

In contrast, the charging and discharging routine for each switchingelement S*# according to the fifth embodiment is configured to set therate of discharging the gate of the switching element S*# to one of alower value and a higher value in the middle of the discharge periodfrom the start of discharging the gate of the switching element S*# tothe completion of the discharge from the gate of the switching elementS*#.

In this embodiment, in place of the disabling flag F, a discharging-ratechanging flag F1 is used. The discharging-rate changing flag F1 beingset to 1 enables the drive controller 22 to set the rate of dischargingthe gate of the switching element S*# to the lower value. Thedischarging-rate changing flag F of 0 enables the drive controller 22 toset the rate of discharging the gate of the switching element S*# to thehigher value.

FIG. 12 schematically illustrates specific operations of the variablescorrelating with the charging and discharging routine when the rate ofdischarging the gate of the switching element S*# is set to the lowervalue according to this embodiment. (a) to (f) of FIG. 12 correspond to(a) to (e), and (g) of FIG. 6. Note that, in FIG. 12, the illustrationof how the sense voltage Vse to be used for comparison with thereference voltage Vref varies as “valid” or “invalid” is omitted becauseit is identical to those in (h) of FIG. 6.

When a drive signal Vi*# inputted to the drive unit DU is changed fromthe off command to the on command at time t51, the charging task for thegate of the switching element S*# is started at the time t51. Thischarging task results in the gate of the switching element S*# startingto rise (see steps S6 and S8 of FIG. 14).

After the start of the increase in the gate voltage Vge, the measuredlevel of the sense voltage Vse to be used for comparison with thethreshold voltage Vth2 is validated at time t52 when the first thresholdtime TA has elapsed since the change of the drive signal Vi*# from theoff command to the on command (see step S12). Thereafter, when it isdetermined that the measured level of the sense voltage Vse is equal toor higher than the threshold voltage Vth2 at time t53 within the periodduring which the measured level of the sense voltage Vse to be used forcomparison with the threshold voltage Vth2 is validated (see YES in stepS14), the discharging-rate changing flag F1 is set to 1 (see step S16).

The discharging-rate changing flag F1 being set to 1 enables the drivecontroller 22 to set the rate of discharging the gate of the switchingelement S*# to the lower value.

Thereafter, when the drive signal Vi*# is changed from the on command tothe off command at time t54 (see YES in step S18), the measured level ofthe sense voltage Vse to be used for comparison with the thresholdvoltage Vth2 is invalidated, and the measured level of the sense voltageVse to be used for comparison with the threshold voltage Vth1 isvalidated (see step S20).

At that time, because the discharging-rate changing flag F1 is 1 so thatthe determination in step S22 is YES, the discharging task is carriedout while the rate of discharging the gate of the switching element S*#is set to the lower value in step S28. Specifically, the gate of theswitching element S*# is discharged via one of the first and secondpositive-charge dissipating paths. This sets the rate of discharging thegate of the switching element S*# to the lower value (see step S28 andthe period from the time t54 to time t55 in FIG. 12).

This discharging task gives a priority to the effect of decreasing asurge in comparison to the effect of reducing switching loss.

FIG. 13 schematically illustrates specific operations of the variablescorrelating with the charging and discharging routine when the rate ofdischarging the gate of the switching element S*# is set to the highervalue according to this embodiment. (a) to (f) of FIG. 13 correspond to(a) to (f) of FIG. 6. Note that, in FIG. 12, the illustration of How thesense voltage Vse to be used for comparison with the reference voltageVref varies as “valid” or “invalid” is omitted because it is identicalto those in (h) of FIG. 6.

When a drive signal Vi*# inputted to the drive unit DU is changed fromthe off command to the on command at time t61, the charging task for thegate of the switching element S*# is started at the time t61. Thischarging task results in the gate of the switching element S*# startingto rise (see steps S6 and S8 of FIG. 14).

After the start of the increase in the gate voltage Vge, the measuredlevel of the sense voltage Vse to be used for comparison with thethreshold voltage Vth2 is validated at time t62 when the first thresholdtime TA has elapsed since the change of the drive signal Vi*# from theoff command to the on command (see step S12). Thereafter, when it isdetermined that the measured level of the sense voltage Vse is kept tobe lower than the threshold voltage Vth2 within the period during whichthe measured level of the sense voltage Vse to be used for comparisonwith the threshold voltage Vth2 is validated (see NO in step S14), thedischarging-rate changing flag F1 is kept to 0 (see the skip of theoperation in step S16).

The discharging-rate changing flag F1 being set to 0 enables the drivecontroller 22 to set the rate of discharging the gate of the switchingelement S*# to the higher value.

Thereafter, when the drive signal Vi*# is changed from the on command tothe off command at time t63 (see YES in step S18), the measured level ofthe sense voltage Vse to be used for comparison with the thresholdvoltage Vth2 is invalidated, and the measured level of the sense voltageVse to be used for comparison with the threshold voltage Vth1 isvalidated (see step S20).

At that time, because the discharging-rate changing flag F1 is 0 so thatthe determination in step S22 is NO, the discharging task is carried outwhile the rate of discharging the gate of the switching element S*# isset to the higher value in step S32. Specifically, the gate of theswitching element S*# is discharged via both the first and secondpositive-charge dissipating paths. This sets the rate of discharging thegate of the switching element S*# to the higher value (see step S32 andthe period from the time t63 to time t64 in FIG. 13).

This discharging task gives a priority to the effect of reducingswitching loss in comparison to the effect of decreasing a surge.

As described above, the drive unit DU for each switching element S*#according to this embodiment is configured to perform the dischargingtask while balancing the effect of decreasing a surge and the effect ofreducing switching loss.

The drive units DU and the control system according to each of the firstto fifth embodiments can be modified.

In each of the first to fourth embodiments, the sense voltage Vse isused as a parameter used to determine whether to perform the task ofdisabling the active gate control, but the present disclosure is notlimited thereto.

Specifically, a parameter as a function of the level of the sensevoltage Vse, such as an output current flowing out of the sense terminalSt of the switching element S*# can be used. In this modification, adetector 60 for detecting the output current from the sense terminal St,which correlates with the collector current flowing through theconductive path of the switching element S*# can be used.

As such a parameter, in addition to a parameter correlating with thecollector current, the collector current itself can be used. Forexample, the detector 60 can be configured to directly detect thecollector current. FIG. 2 schematically illustrates the detector 60,such as a hall element, configured to directly detect the collectorcurrent. The detector 60 can be eliminated from the control system 100according to the first embodiment. In this modification, as illustratedin, for example, FIG. 4, the collector current may be unstable withinthe period during which the switching element S*# is changed from OFF toON. For this reason, as described in the first embodiment, a level ofthe collector current detected by the detector 60 after the firstthreshold time TA has elapsed since the change of the drive signal Vi*#from the off command to the on command is preferably used to determinewhether to perform the task of disabling the active gate control. Theusing of the level of the collector current after the lapse of the firstthreshold time TA makes it possible to determine whether there is a needto perform the task of disabling the active gate control with highaccuracy.

Similarly, in the fifth embodiment, the output current flowing out ofthe sense terminal St of the switching element S*# or the collectorcurrent can be used to determine whether the rate of discharging thegate of the switching element S*# is changed between the lower level andthe higher level.

In each of the first to fifth embodiments, the drive IC 20, moreparticularly, the drive controller 22 is configured to perform thecharging and discharging routine, but another component, such as thecontrol unit 14, can be configured to perform the charging anddischarging routine. In this modification, for example, in each of theoperations in steps S10, S18, and S30 illustrated in FIG. 5, the drivesignal g*# can be used in place of the drive signal Vi*#. In otherwords, a driver according to the present disclosure can be designed as adrive unit DU for a switching element S*#, the combination of the driveunit DU and another component configured to perform the charging anddischarging routine.

In the first embodiment, the drive controller 22 changes the rate ofdischarging the gate of the switching element S*# from the higher valueto the lower value at the time t2A in FIG. 3 corresponding to the timet7 in FIG. 6 at which the measured level of the sense voltage Vsereaches the threshold voltage Vth1, but the present disclosure is notlimited thereto. Specifically, the drive controller 22 can change therate of discharging the gate of the switching element S*# from thehigher value to the lower value at a time earlier than the time t2A atwhich the measured level of the sense voltage Vse reaches the thresholdvoltage Vth1 as long as the time earlier than the time t2A is within theperiod during which the drive signal Vi*# is the off command. Note thatthe earlier the time to change the rate of discharging the gate of theswitching element S*# from the higher value to the lower value is, thelower the switching speed of the switching element S*# is.

In each of the first to fifth embodiments, the drive controller 22changes the rate of discharging the gate of the switching element S*#using two different values, but the present disclosure can change therate of discharging the gate of the switching element S*# using three ormore values. For example, the drive controller 22 can change the rate ofdischarging the gate of the switching element S*# in three values. Thatis, the drive controller 22 can change the discharge path of the gate ofthe switching element S*# among: the first positive-charge dissipatingpath including the resistor 34 a; the second positive-charge dissipatingpath including the resistor 34 b different in resistance than theresistor 34 a; and the high-resistance discharge path including thesoft-turnoff switching element 44.

In the first embodiment, the drive controller 22 disables the measuredlevel of the sense voltage Vse to be used for comparison with thereference voltage Vref within the first period until the first thresholdtime TA has elapsed since the change of the drive signal Vi*# from theoff command to the on command, but the present disclosure is not limitedthereto. Specifically, the drive controller 22 can disable the measuredlevel of the sense voltage Vse to be used for comparison with thereference voltage Vref within a period until the measured level of thesense voltage Vse reaches the preset voltage level Va.

In the fifth embodiment, the drive controller 22 uses the approachdisclosed in the first embodiment to disable the measured level of thesense voltage Vse to be used for comparison with the threshold voltageVth2, but the present disclosure is not limited thereto. Specifically,the drive controller 22 can use one of the approaches disclosed in therespective second to fourth embodiments to disable the measured level ofthe sense voltage Vse to be used for comparison with the thresholdvoltage Vth2.

In each of the first to fifth embodiments, an IGBT is used as aswitching element S*# of each drive unit DU, but an N-channel MOSFET ora P-channel MOSFET can be used as a switching element S*# of each driveunit DU. If a P-channel MOSFET is used as a switching element S*# ofeach drive unit DU, because a potential difference from the on-offcontrol terminal, i.e. the gate, of the P-channel MOSFET to one end,i.e. the source, thereof is set to be a negative value turns theP-channel MOSFET to on state. Thus, in this case, the drive controller22 stores a positive charge on the gate of the P-channel MOSFET, i.e.the switching element S*#, to turn off the P-channel MOSFET.

In each of the first to fifth embodiments, a switching element S*# towhich a drive unit DU according to a corresponding one of theseembodiments is provided is installed in each of the inverter INV and theconverter CNV. However, in the present disclosure, a switching elementS*# to which a drive unit DU according to a corresponding one of theseembodiments is provided can be installed in another device. A powerconverter, such as an inverter and a converter, in which a switchingelement S*# to which a drive unit DU according to a corresponding one ofthese embodiments is provided is installed is installed in a motorvehicle, but can be installed in another machine.

While illustrative embodiments of the present disclosure have beendescribed herein, the present disclosure is not limited to theembodiments described herein, but includes any and all embodimentshaving modifications, omissions, combinations (e.g., of aspects acrossvarious embodiments), adaptations and/or alternations as would beappreciated by those in the art based on the present disclosure. Thelimitations in the claims are to be interpreted broadly based on thelanguage employed in the claims and not limited to examples described inthe present specification or during the prosecution of the application,which examples are to be construed as non-exclusive.

What is claimed is:
 1. A driver for selectively performing a chargingtask and a discharging task for an on-off control terminal of avoltage-controlled switch having a conductive path, according to a drivesignal directing selectively an on state and an off state of thevoltage-controlled switch, thus setting the voltage-controlled switch toone of the on state and the off state, the driver comprising: adischarging-rate changing module adapted to change a rate of dischargingthe on-off control terminal of the voltage-controlled switch at leastbetween a first value and a second value lower than the first value; ameasuring module configured to measure a value of a parameter as afunction of a current flowing through the conductive path of thevoltage-controlled switch during the drive signal being in the on state;and a discharging control module configured to: control thedischarging-rate changing module, as a function of the value of theparameter, to select one of the first value and the second value as therate of discharging the on-off control terminal of thevoltage-controlled switch upon the drive signal directing a change fromthe on state of the voltage-controlled switch to the off state thereof;and discharge the on-off control terminal of the voltage-controlledswitch using the selected one of the first value and the second value asthe rate of discharging the on-off control terminal of thevoltage-controlled switch.
 2. The driver according to claim 1, whereinthe discharging control module is configured to: if the value of theparameter measured by the measuring module, after the voltage-controlledswitch is in the on state, is lower than a first threshold value,control the discharging-rate changing module to select the first valueas the rate of discharging the on-off control terminal of thevoltage-controlled switch within at least a period during which thevalue of the parameter measured by the measuring module is rising whilethe drive signal is directing the voltage-controlled switch to be in theoff state.
 3. The driver according to claim 1, wherein the dischargingcontrol module is configured to: if the value of the parameter measuredby the measuring module, after the voltage-controlled switch is in theon state, is equal to or higher than a first threshold value, controlthe discharging-rate changing module to change the rate of dischargingthe on-off control terminal of the voltage-controlled switch from thefirst value to the second value within a period during which the valueof the parameter is rising while the drive signal is directing thevoltage-controlled switch to be in the off state; and if the value ofthe parameter measured by the measuring module, after thevoltage-controlled switch is in the on state, is lower than the firstthreshold value, control the discharging-rate changing module to selectthe first value as the rate of discharging the on-off control terminalof the voltage-controlled switch within a period during which the drivesignal is directing the voltage-controlled switch to be in the offstate.
 4. The driver according to claim 3, wherein: thevoltage-controlled switch includes: a pair of high- and low-sideswitching elements connected in series and connected to positive andnegative terminals of a DC power source, each of the high- and low-sideswitching elements having a sense terminal from which a currentcorrelating with the current flowing through the conductive path isoutputted; and a pair of flywheel diodes, each of which is connected inantiparallel to a respective one of the high- and low-side switchingelements, the measuring module is configured to measure the value of theparameter as a function of the current outputted from the sense terminalof each of the high- and low-side switching elements, and thedischarging control module is configured to: determine a timing at whichthe value of the parameter measured by the measuring module becomesequal to or higher than a second threshold value as a timing to changethe rate of discharging the on-off control terminal of each of the high-and low-side switching elements from the first value to the secondvalue, the first threshold value being lower than the second thresholdvalue; and validate the value of the parameter measured by the measuringmodule to be used for determining the timing to change the rate ofdischarging the on-off control terminal of each of the high- andlow-side switching elements within only a period during which the drivesignal is directing the voltage-controlled switch to be in the offstate.
 5. The driver according to claim 1, wherein the dischargingcontrol module is configured to: if the value of the parameter measuredby the measuring module, after the voltage-controlled switch is in theon state, is lower than a first threshold value, control thedischarging-rate changing module to select the first value as the rateof discharging the on-off control terminal of the voltage-controlledswitch within a period during which the drive signal is directing thevoltage-controlled switch to be in the off state; and if the value ofthe parameter measured by the measuring module, after thevoltage-controlled switch is in the on state, is equal to or higher thanthe first threshold value, control the discharging-rate changing moduleto select the second value as the rate of discharging the on-off controlterminal of the voltage-controlled switch within a period during whichthe drive signal is directing the voltage-controlled switch to be in theoff state.
 6. The driver according to claim 1, wherein the dischargingcontrol module is configured to: compare the value of the parametermeasured by the measuring module with a first threshold value todetermine whether the value of the parameter is lower than the firstthreshold value; and if a preset time has elapsed since a timing atwhich the drive signal directing the voltage-controlled switch to be inthe on state occurs, validate the value of the parameter measured by themeasuring module to be used for the comparison with the first thresholdvalue within a period during which the drive signal is directing thevoltage-controlled switch to be in the on state.
 7. The driver accordingto claim 2, wherein the discharging control module is configured to:compare the value of the parameter measured by the measuring module withthe first threshold value to determine whether the value of theparameter is lower than the first threshold value; and if a preset timehas elapsed since a timing at which the drive signal directing thevoltage-controlled switch to be in the on state occurs, validate thevalue of the parameter measured by the measuring module to be used forthe comparison with the first threshold value within a period duringwhich the drive signal is directing the voltage-controlled switch to bein the on state.
 8. The driver according to claim 1, wherein thedischarging control module is configured to: compare the value of theparameter measured by the measuring module with a first threshold valueto determine whether the value of the parameter is lower than the firstthreshold value; and if a voltage at the on-off control terminal of thevoltage-controlled switch is equal to or higher than a thresholdvoltage; and the voltage-controlled switch has been completely changedto be in the on state, validate the value of the parameter measured bythe measuring module to be used for the comparison with the firstthreshold value.
 9. The driver according to claim 2, wherein thedischarging control module is configured to: compare the value of theparameter measured by the measuring module with the first thresholdvalue to determine whether the value of the parameter is lower than thefirst threshold value; and if a voltage at the on-off control terminalof the voltage-controlled switch is equal to or higher than a thresholdvoltage; and the voltage-controlled switch has been completely changedto be in the on state, validate the value of the parameter measured bythe measuring module to be used for the comparison with the firstthreshold value.
 10. The driver according to claim 1, wherein thedischarging control module is configured to: compare the value of theparameter measured by the measuring module with a first threshold valueto determine whether the value of the parameter is lower than the firstthreshold value; and upon the drive signal directing a change from theon state of the voltage-controlled switch to the off state thereof,validate the value of the parameter measured by the measuring module tobe used for the comparison with the first threshold value.
 11. Thedriver according to claim 2, wherein the discharging control module isconfigured to: compare the value of the parameter measured by themeasuring module with the first threshold value to determine whether thevalue of the parameter is lower than the first threshold value; and uponthe drive signal drive signal directing a change from the on state ofthe voltage-controlled switch to the off state thereof, validate thevalue of the parameter measured by the measuring module to be used forthe comparison with the first threshold value.
 12. The driver accordingto claim 1, wherein the discharging control module is configured to:compare the value of the parameter measured by the measuring module witha first threshold value to determine whether the value of the parameteris lower than the first threshold value; and if a preset reference timehas elapsed since a timing at which the value of the parameter measuredby the measuring module became equal to or higher than the firstthreshold value, validate the value of the parameter measured by themeasuring module to be used for the comparison with the first thresholdvalue.
 13. The driver according to claim 2, wherein the dischargingcontrol module is configured to: compare the value of the parametermeasured by the measuring module with the first threshold value todetermine whether the value of the parameter is lower than the firstthreshold value; and if a preset reference time has elapsed since atiming at which the value of the parameter measured by the measuringmodule became equal to or higher than the first threshold value,validate the value of the parameter measured by the measuring module tobe used for the comparison with the first threshold value.
 14. Thedriver according to claim 6, wherein the voltage-controlled switchincludes: a pair of high- and low-side switching elements connected inseries and connected to positive and negative terminals of a DC powersource, each of the high- and low-side switching elements having a senseterminal from which a current correlating with the current flowingthrough the conductive path is outputted; and a pair of flywheel diodes,each of which is connected in antiparallel to a respective one of thehigh- and low-side switching elements, and the measuring module isconfigured to measure the value of the parameter as a function of thecurrent outputted from the sense terminal of each of the high- andlow-side switching elements.
 15. The driver according to claim 7,wherein the voltage-controlled switch includes: a pair of high- andlow-side switching elements connected in series and connected topositive and negative terminals of a DC power source, each of the high-and low-side switching elements having a sense terminal from which acurrent correlating with the current flowing through the conductive pathis outputted; and a pair of flywheel diodes, each of which is connectedin antiparallel to a respective one of the high- and low-side switchingelements, and the measuring module is configured to measure the value ofthe parameter as a function of the current outputted from the senseterminal of each of the high- and low-side switching elements.
 16. Thedriver according to claim 14, further comprising: a turnoff moduleconfigured to discharge the on-off control terminal of each of the high-and low-side switching elements using a third value as the rate ofdischarging the on-off control terminal of a corresponding one of thehigh- and low-side switching elements upon the value of the parametermeasured by the measuring module being equal to or higher than areference value, the third value being lower than the second value; anda disabling module configured to disable the turnoff module fromdischarging the on-off control terminal of each of the high- andlow-side switching elements within at least one of: a first period untila reference time has elapsed since a timing at which the drive signaldirected a change to the on state of the voltage-controlled switch; anda second period during which the drive signal is directing thevoltage-controlled switch to be in the off state.
 17. The driveraccording to claim 15, further comprising: a turnoff module configuredto discharge the on-off control terminal of each of the high- andlow-side switching elements using a third value as the rate ofdischarging the on-off control terminal of a corresponding one of thehigh- and low-side switching elements upon the value of the parametermeasured by the measuring module being equal to or higher than areference value, the third value being lower than the second value; anda disabling module configured to disable the turnoff module fromdischarging the on-off control terminal of each of the high- andlow-side switching elements within at least one of: a first period untila reference time has elapsed since a timing at which the drive signaldirected a change to the on state of the voltage-controlled switch; anda second period during which the drive signal is directing thevoltage-controlled switch to be in the off state.
 18. A control systemfor controlling a rotary machine, the control system comprising: aconverter equipped with at least one pair of first voltage-controlledswitching elements connected in series, each of the firstvoltage-controlled switching elements having a conductive path and anon-off control terminal; and an inverter equipped with at least one pairof second voltage-controlled switching elements connected in series,each of the second voltage-controlled switching elements having aconductive path and an on-off control terminal; and a driver forselectively performing a charging task and a discharging task for theon-off control terminal of each of the voltage-controlled switchingelements, according to a drive signal directing selectively an on stateand an off state of the voltage-controlled switch to thereby boost a DCvoltage inputted to the converter, and invert the boosted DC voltageinto an AC voltage to be supplied to the rotary machine, the driver foreach of the first and second voltage-controlled switching elementscomprising: a discharging-rate changing module adapted to change a rateof discharging the on-off control terminal of a corresponding one of thefirst and second voltage-controlled switching elements at least betweena first value and a second value lower than the first value; a measuringmodule configured to measure a value of a parameter as a function of acurrent flowing through the conductive path of a corresponding one ofthe first and second voltage-controlled switching elements during thedrive signal being in the on state; and a discharging control moduleconfigured to: control the discharging-rate changing module, as afunction of the value of the parameter, to select one of the first valueand the second value as the rate of discharging the on-off controlterminal of a corresponding one of the first and secondvoltage-controlled switching elements upon the drive signal directing achange from the on state of a corresponding one of the first and secondvoltage-controlled switching elements to the off state thereof; anddischarge the on-off control terminal of a corresponding one of thefirst and second voltage-controlled switching elements using theselected one of the first value and the second value as the rate ofdischarging the on-off control terminal thereof.